Keyboard Controlled submenu items

I have been approached by people who are blind or very visually impaired. They navigate through the website using the keyboard. The websites that contain a submenu are not accessible to them. They cannot access the submenu items without a mouse. Since they can't use a mouse, they can't come here. Is there perhaps (already) a key combination for this? They can use the TAB key to navigate through the menu, but not to sub-items.